A confocal scanning microscope is known as a technique to increase resolving power of an optical microscope (see for example, C. J. R. Sheppard and A. Choudhury, “Image Formation in the Scanning Microscope,” Opt. Acta, Vol. 24, 1051-1073 (1977)). An optical system thereof may be of a reflection type or a transmission type. In a following description, for ease of understanding, it is assumed that the optical system is of the transmission type. FIG. 3 is a schematic diagram of a transmission-type optical system. Light emitted from a light source 101 illuminates a pinhole 205. As a result, a point light source is obtained. In practice, the pinhole has a finite size, and thus the obtained point light source is an approximation to an ideal pinhole. Light emerging from the pinhole is focused by an objective lens 201 onto an object 202 under observation. The object 202 under observation can be three-dimensionally scanned using a scanning mechanism 102 such as a voice coil. After passing through the object under observation, the light further passes through an objective lens 203, which focuses the light onto a pinhole 204. The light passing through the pinhole 204 is detected by a photodetector 103, which outputs a signal corresponding to the incident light. The output signal from the photodetector 103 is displayed on a display apparatus 104 such that an image of the object under observation corresponding to a scanning position is displayed. It is known that resolving power in a lateral direction of the confocal scanning microscope depends on the pinhole 204. The resolving power can be increased by reducing the pinhole size. Conversely, if the pinhole size is increased, the result is degradation in the resolving power, and the resolving power can become similar to that of a non-scanning-type optical microscope configured such that a large area of an object under observation is illuminated and reflected light from the object under observation is passed through an objective lens to form an image of the object under observation. FIG. 4 illustrates a point spread function obtained in a confocal scanning microscope with a pinhole with a minimized size and also illustrates a point spread function obtained in an optical microscope of a non-scanning type. A horizontal axis v is expressed in normalized optical units. More specifically, v is defined as v=2π×x×NA/λ. In this definition, NA denotes the numerical aperture of the objective lens 201 and the 203. Note that it is assumed that both lenses have equal NA. In the definition, x denotes a coordinate in a direction (a lateral direction) perpendicular to an optical axis, and λ denotes a wavelength of light. As can be seen from FIG. 4, the point spread function 131 of the confocal scanning optical microscope is narrower than the point spread function 130 of the non-scanning-type optical microscope, which means that the confocal scanning optical microscope provides higher resolving power.
It is possible to increase the resolving power of the confocal scanning optical microscope by reducing the pinhole size as described above. However, the reduction in the pinhole size results in a reduction in intensity of light to be detected, and thus degradation in signal-to-noise ratio occurs. One technique to deal with the above problem is to use an optical interference effect as disclosed, for example, in K. Wicker and R. Heintzmann, “Interferometric Resolution Improvement for Confocal Microscopes,” Optics Express, Vol. 15, No. 19, 12206-12216 (2007). FIG. 5 schematically illustrates an optical system configured according to this technique. Laser light emitted from a laser light source 101 is converted into collimated light by a collimator lens 206 and then reflected by a beam splitter 207 The reflected laser light is incident on an objective lens 201. The objective lens 201 focuses the laser light onto an object 202 under observation which can be scanned by a scanning mechanism 102. Laser light reflected by the object 202 under observation returns to the objective lens 201 and passes through the beam splitter 207. After passing through the beam splitter 207, the laser light is split by a beam splitter 208 into two laser beams. One of the two laser beams passes through the beam splitter 208 and is reflected by a reflecting mirror 211. This laser beam passes through a phase retarder 212 and is then incident on a beam splitter 213. On the other hand, the other one of the two laser beams is reflected by the beam splitter 208 and passes through two convex lenses 209 and 210. When the laser beam passes through the two convex lenses 209 and 210, an image of the beam is inverted with respect to the optical axis thereof. The laser beam is then reflected by a reflecting mirror 216 and is incident on the beam splitter 213.
Thus, the beam splitter 213 receives the two laser beams 251 and 252 which are incident from two directions. Each of the two laser beams 251 and 252 is split by the beam splitter 213 into reflected light and transmitted light. Thus, interfering light emerges in two directions, and these light beams are focused by condensing lenses 214 and 215 onto detectors 105 and 106, respectively. Pinholes 217 and 218 are disposed on the respective detectors 105 and 106 to limit detection areas thereof. Let θ1 denote the phase of the laser beam 252 passing through the beam splitter 213, θ2 denote the phase of the laser beam 252 reflected by the beam splitter 213, φ1 denote the phase of the transmitted laser beam 251, and φ2 denote the phase of the reflected laser beam 251. Note that these phases have a relation θ2−φ1=θ1−φ2+π. If there is no phase difference between laser beams 251 and 252, and if the phases are denoted by 0° (θ1=φ1=0) , it is possible to achieve θ2 =π and φ2=0 by properly selecting the material of the beam splitter 213. In this state, it is possible to adjust the phase retarder 212 such that one of the interfering laser beams incident on the respective photodetectors 105 and 106 is in a constructive state and the other one is in a disconstructive state. By determining the difference between these two interfering laser beams, it is possible to obtain only an interfering light component, and it is possible to achieve a high-resolution image using a signal of the interfering light component . In this technique, the pinhole size is not so small as that of the confocal scanning microscope, a significant reduction in the light intensity occurs, which is a benefit provided by this technique.